Recently, provision of higher operating performance, higher operating function, and further down-sizing has rapidly been promoted for electronic apparatuses, and accordingly, there is a greater demand for further down-sizing and lighter weight of electronic parts used for electronic apparatuses. Likewise, there is a strong demand for higher thermal resistance, higher mechanical strength, and higher electric characteristics of materials used for composing electronic parts. In addition, higher density, higher operating function and higher operating performance characteristics are demanded for the method of packaging semiconductor elements as well as for printed circuit boards for mounting them.
For example, against conventional hard printed circuit boards for loading electronic parts, flexible printed circuit boards have drawn attention of the concerned to result in the sharp increase of the demand for them. In the field of packaged semiconductor modules, for example, a variety of structures such as COL (chip on lead) for loading a chip on a lead frame devoid of die-pad or LOC (lead on chip) for loading lead wires on a chip have been proposed relative to materialization of higher functional capability and greater capacity, and these devices have actually been offered for practical service. To satisfy such requirements, wire-thinning and formation of multi-layers have been executed for flexible printed circuit boards (FPC) to result in the debut of such FPCs for mounting electronic parts, or bilateral FPCs, or such extremely dense multi-layer FPCs. This in turn results in the demand for higher performance capability of insulation adhesive agent and insulation organic film. In particular, in the case of high-density-loading materials such as Loc packages or MCMs (multi-chip module) or print-wiring material such as multi-layer FPCs or in the case of using them for composing space-aviation material, it is essential that usable materials contain extremely high thermal resistant property and mechanical strength and be capable of exerting distinguished workability and adhesive property, particularly it is required that usable materials be extremely low in moisture absorbent property, and yet, be combined with essential characteristics such as dielectric property and dimensional stability.
For the present, such films composed of polyimide resin having high thermal resistant property and mechanical strength and outstanding electric characteristic have preferentially been used as organic insulating material for making up base film or cover film of FPCs. However, normally, polyimide resin itself is substantially insoluble and infusible in the closed cycle condition. Since polyimide resin is thermally infusible, it is devoid of adhesive property. Accordingly, adhesive agent is used for adhesion and covering purpose. In many cases, because of satisfactory processable property under low temperature range (below 180.degree. C.) and workability, epoxy resin and acrylic resin are respectively used for insulating adhesive agent. On the other hand, generally, epoxy adhesive agent is mainly used for adhering a cover-lay film onto circuit surface of a flexible printed circuit board. However, it is essential that a perforation process be executed against the cover-lay film before adhering the film onto the flexible printed circuit board. Conventionally, positioning of the cover-lay film against the flexible printed circuit board has substantially been performed via manual work to result in the poor workability and increased cost. And yet, both of the above-cited adhesive agents have proved to be inferior to polyimide resin in terms of thermal resistant property as a result of incurring deterioration at a high temperature range (250.degree. C.), thus raising problem in that said conventional adhesive agents failed to fully activate proper characteristics of polyimide resin used for the base film of FPCs.
In the case of a semiconductor module comprising COL or LOC structure for example, the semiconductor chip is secured to a lead frame with adhesive agent. Outstanding adhesive property is required for the usable adhesive agent without causing interface between the chip and the lead frame to be separated when moisture is absorbed and whenever the chip and the lead frame are subject to thermal stress caused by reflow-soldering or temperature cycle. From the viewpoint of facilitating mass production, it is desired that adhesion be achieved in the shortest period of time, and yet, based on the usable form, it is desired that adhesion be implemented at a relatively low temperature range. There is such an incidental case in which package crack or burst may be generated by water absorbed in the adhesive agent layer in the package structure after being steamed by soldering heat. To prevent incidental package crack or burst from occurrence, it is required that moisture absorbent property of adhesive agent layer be as low as possible. In addition, there are a variety of characteristics being required for usable adhesive agent, and thus, selection of adhesive agent makes up an important issue in the production of semiconductor modules.
However, dielectric characteristic and moisture absorbent property of conventional adhesive agents in use respectively fail to fully satisfy high standard demanded for electronic parts, and thus, it is pointed out that conventional adhesive agents will not properly be applicable to high-density semiconductor module packages to be assembled in the future. Furthermore, since conventional reactive thermosetting resins including epoxy resin respectively need to be cured at a high temperature for a long duration, all of said resins are apt to induce failure of electric/electronic apparatuses, and thus, these resins are not suited for composing electric/ electronic parts. In addition, conventional reactive thermosetting resins respectively need a post-curing process that lasts for a long while. Consequently, in order to produce electric/elelctronic parts to be mounted on FPCs with high density, provision of higher-performance adhesive agent has been demanded.
Recently, semiconductor modules incorporating a variety of LOC structures or COL structures have been developed. For example, the Japanese Laid-Open Patent Publication HEI-3-12781 discloses such a semiconductor module incorporating LOC structure. However, said Patent Publication merely describes that adhesive-agent layer is selected from epoxy resin, acrylic resin, silicone resin, or polyimide resin, without concrete exemplification. It is thus inferred that the above prior art merely uses any of conventionally known general-purpose adhesive agents without improving characteristic of adhesive-agent layer.
On the other hand, fluorocarbon resin such as FEP has widely been used as one of thermoplastic resins for providing adhesive property. However, because of extremely poor resistivity against radioactive rays, fluorocarbon resin is not suited for specific uses requiring resistivity against radioactive rays. Under the above circumstances, an early development of novel film incorporating distinct resistivity against radioactive rays and chemicals, distinct characteristic under low temperature, distinct adhesive property, and distinct thermally fusible adhesive property with satisfactory dielectric characteristic and lower water-absorbent property, has long been desired. In the case of fabricating such a tubular film made from polyimide resin applicable to a variety of uses such as FPCs, cable insulation film, medical instruments, or the like, normally, edges of produced film are superposed by cylindrically folding it, and then edges are adhered with adhesive agent to make up tubular form. However, due to restrictive factor in the process of producing polyimide film via casting method, polyimide film is expensive, and yet, juncture remains on the film. In addition, when adhesion is improperly effected or adhesive agent generates degradation, film edges are disengageable along juncture to raise a problem. In particular, when using adhesive agent, resistivity of polyimide film against heat and chemicals is restricted by characteristic of adhesive agent to result in the failure to fully exert proper characteristic of polyimide resin.
Recently, in conjunction with development of elementary particle physics, construction of such an accelerator for generating extremely high energy has been underway. In order to generate such extremely high energy, provision of magnets capable of generating intense magnetism by feeding huge current is essential. As a recent trend, there are increasing cases in which superconductive magnets using superconductive wires are introduced.
To compose superconductive wires, in many cases, oxide mainly comprising copper is used. However, when using adhesive agent made from thermosetting resin in the course of covering superconductive wires with insulation cover material, thermal treatment causes rate of oxidizing superconductive wires to vary to result in the degraded characteristic of magnets. Accordingly, it is essential for the above process that such an adhesive agent capable of curing and properly achieving adhesion under low temperature be used.
Because of operating characteristic, the above accelerator generates a huge volume of radioactive rays. In order to more effectively utilize magnetism, recently, superconductive magnets are set up at a location closer to tubes for transferring elementary particles, and consequently, superconductive magnets are readily affected by radioactive rays. Accordingly, it is essential that distinguished resistivity against radioactive rays be provided for insulation cover material and adhesive agent used for superconductive magnets.
For covering superconductive wires under extremely low temperature, laminate of thermosetting resin mainly comprising epoxy resin built on polyimide film has been used. However, since curing of epoxy resin requires high-temperature treatment for a long period of time, superconductive wires are subject to deterioration. On the other hand, since epoxy resin does not exert sufficient resistivity against radioactive rays, and yet, in anticipation of further increase in the volume of radioactive rays to be generated in conjunction with further growth of energy to be emitted from accelerators, provision of novel adhesive agent capable of achieving adhesion at low temperature and exerting distinct resistivity against radioactive rays has been demanded.
In order to store extremely low-temperature material such as liquefied helium, conventionally, such a container comprising double-wall structure consisting of an inner vessel and an outer vessel interspaced via vacuum filled with insulation material has been used. To make up insulation material, such a laminate sheet comprising a reflection film comprising a polyester film or a polyimide film having a single surface or both surfaces being adhered with an aluminium foil(s) or adhered with evaporated aluminium and a spacer comprising a glass-fiber cloth or net composed of plastic yarns such as polyester yarns has been used.
However, recently, in conjunction with rapid progress in the art of utilizing extremely low temperature requiring liquefied helium and in the art of utilizing superconductor, form of refrigerated objects has been complicated, and yet, space for setting laminated insulation material tends to be contracted. In addition to thermal insulation property, a variety of characteristics are required for insulation material to be used. For instance, in the field of nuclear fusion or an accelerator, in order to generate intense magnetism, use of superconductive magnets is essential, and thus, strength and resistivity against radioactive rays are required for such insulation material. Nevertheless, when using general-purpose resins such as polyester resin for forming reflection films, the film for making up laminated insulation material is affected by irradiated radioactive rays to incur degradation and deformation to result in the failure to hold sufficient thermal insulation property. Furthermore, since effect of refrigerating agent is lowered when impurities such as water absorbed in the insulation material intermix with refrigerating agent such as liquefied helium, the insulation material is previously heated to remove impurities via evaporation. However, since polyester film is poor in thermal resistant property, it cannot be treated with sufficient high temperature, thus raising problems such as prolonged duration of thermal treatment and residue of impurities remaining thereon.
In the case of using such a film incorporating satisfactory thermal resistant property and resistivity against radioactive rays made from curable polyimide film ("APICAL", a product and a registered trade name of Kanegafuchi Chemical Industrial Co., Ltd.) for example for the above reflection plastic film, since the curable polyimide film itself is devoid of adhesive property, use of adhesive agent is required. Accordingly, own performance characteristic is affected by adhesive agent to be used, and yet, since there is no adhesive agent having distinct thermal resistance property and resistivity against radioactive rays, only negligible effect is yielded from the use of polyimide film.
In the course of covering superconductive wires, the wires can be prevented from incurring degradation by using such an adhesive agent composed of low-temperature-curable polyimide resin as the one capable of achieving adhesion at low temperature and being distinct in resistivity against radioactive rays. However, in this case, since curing effect under normal temperature can be promoted at a relatively fast rate, shell life of adhesive agent in stage B is short. This in turn prevents said curable polyimide film in the form of a laminated film from being offered to marketable end uses.
Likewise, in conjunction with development of a variety of electronic apparatuses, enhancement of performance characteristic has also been required for capacitors built in them. Normally, general-purpose resins such as polyethylene resin or polypropylene resin are used for composing dielectric film of plastic capacitors. However, any of said general-purpose resins have proved to be poor in thermal resistant property, and accordingly, any of conventional capacitors using general-purpose resins is poor in thermal resistant property to result in the limited uses.
To cope with the above-described circumstances by way of facilitating development of thermally fusible adhesive laminate films for covering wires, materials for composing novel adhesive agent applicable to electronic modules such as reflection film of laminated insulation material and dielectric film for composing capacitors, and yet, in order to properly deal with a variety of uses in other fields, development of novel film and tube featuring easy processability, enhanced thermal resistant property, enhanced chemical resistant property, and reliable adhesive property, has been longed for.
As one of practical methods for solving problems existing in the above-described fields, there is a proposal for polyimide resin incorporating aliphatic radical as applicable to film material having thermal adhesive property. The proposed polyimide resin is inexpensive, and yet, contains practical softening temperature, satisfactory thermal fusible adhesive property, thermal resistant property, and satisfactory chemical resistant property. On the other hand, since molecular weight of polymer is extremely low, the film is devoid of self-supporting property to merely yield fragile structure thereof. Consequently, it is quite difficult to strip the film off from a support body in the course of the casting process. Conversely, while executing an imidizing process by sufficiently heating the film on a support body, the film tightly adheres to the support body to make it impossible to peel both from each other, and thus, the proposed polyimide could hardly be used for the above purpose.
On the other hand, among aromatic polyimide films, non-thermoplastic imide resin and thermosetting imide resin are generally insoluble and infusible, and thus, processing cannot be executed in the form of polyimide. Even in the case of thermoplastic imide resin, because of high viscosity of fused resin, it is necessary to make use of a specific molding machine using high processing temperature. For example, molding temperature of polyether imide ("ULTEM", a product and a registered trade name of GE Japan, Ltd.) is extremely high as it ranges from 340.degree. C. to 425.degree. C. Accordingly, this polyether imide cannot be molded by applying any conventional molding machine, and thus, a specially designed molding machine using high molding temperature is needed. As another processing method, like the case of processing non-thermoplastic imide resin, as mentioned earlier, generally, a thin film is formed from solution of polyamide acid being a precursor of polyimide via a casting process followed by a drying process and an imidizing process to generate a film. Nevertheless, because of high water-absorbent property of this film, there are merely limited uses as a disadvantageous factor thereof.
Recently, there is a proposal on an example of using a polyimide insulation adhesive agent applicable to a copper-coated laminate sheet usable for base film or cover-lay film of said FPC board. For example, according to Japanese Laid-Open Patent Publication HEI-2-138789, a proposal was presented on a method of manufacturing an FPC board for adhering a copper foil to a substrate comprising a polyimide film by using an adhesive film yielded from resinous composition comprising a mixture of aromatic polyimide yielded from 3'3,4'4-benzophenontetracarbonic acid dianhydride and aromatic diamine and polymaleimide. Japanese Laid-Open Patent Publications HEI-5-179224 and HEI-5-112768 respectively made proposals on a variety of adhesive materials made from thermoplastic polyimide resin capable of achieving adhesion via a thermal treatment and a pressing process. Furthermore, recently, even in the field of electronic module, polyimide adhesive agent exerting satisfactory thermal resistant property has been developed.
Any of the above-referred polyimide adhesive agents have proved to be satisfactory in fluidity in the molten condition, exhibited distinct adhesive property via thermocompression bonding, and yet, proved to be distinctly satisfactory in thermal resistant property to make it possible to fully exert characteristic of polyimide film used for the base film of flexible printed circuit boards. However, the above-referred polyimide adhesive agents respectively have proved to generate a problem that adhesion could not be achieved unless being treated with a heating process using a minimum of 300.degree. C .
Furthermore, the above-referred polyimide adhesive agents respectively have proved that these adhesive agents readily absorbed moisture from atmospheric water to result in degraded electric characteristic during storage, and thus they could hardly be stored as of polyimide condition. Accordingly, any of the above-referred polyimide adhesive agents could not be supplied in the state of adhesive film sheet, and consequently, when being used for adhesive agent, solution of polyamide acid being a precursor thereof is initially applied onto an insulation film which is used for the base film or the cover-lay film via a coating process, and then, after being dried, coated polyamide acid is thermally imidized before eventually forming a layer of adhesive agent. Since the above complex processes are needed, a novel method for simplifying production of flexible printed circuit boards has been sought.
In the course of producing semiconductor modules, semiconductor chips may be subject to defect by effect of a heating process on the way of adhesion to result in the lowered productivity. Furthermore, since the layer of adhesive agent absorbs moisture, water is evaporated by heat generated by a soldering process to cause packages to generate crack or burst, and thus, any of conventional adhesive agents cannot readily be used for semiconductor modules. There are such cases in which a layer of adhesive agent absorbs water in the course of manufacturing flexible printed circuit boards to result in the lowered electric characteristic of the adhesive-agent layer, thus eventually deteriorating quality of final products.
Japanese Laid-Open Patent Publications HEI-6-261217 and HEI-6-291152 respectively have disclosed a lead-frame usable for semiconductor modules, which respectively describe that thermoplastic polyimide is used for composing a layer of adhesive agent. Nevertheless, it is inferred that the thermoplastic polyimide specified in said inventions is conventionally known. According to Japanese Laid-Open Patent Publication HEI-6-261217, it is arranged to eliminate influence of remaining distortion caused by heat even though high temperature is necessarily applied to effect adhesion by way of matching thermal expansion coefficients of respective component materials when being built in a package structure. According to Japanese Laid Open Patent Publication HEI-6-291152, a layer of adhesive agent is formed by laminating thermoplastic polyimide and thermoplastic polyether amide imide to cause thermoplastic polyether amide imide to be melted to effect adhesion of a semiconductor chip to the layer of adhesive agent.
However, insofar as such a conventionally known adhesive agent is used, packages cannot be prevented from incurring crack in all cases to result in the failure to fundamentally improve reliability of semiconductor modules. Accordingly, trouble-free material for composing adhesive agent like the one having distinct thermal fusible adhesive property, satisfactory adhesion, least water-absorbent property, and being free from incurring crack to packages, has also been demanded for application to electronic modules.
In the structure of plastic capacitors, thermally resistant resinous film such as polyimide film is used for specific portion where distinct thermal resistant property is essentially required. Nevertheless, since conventional polyimide film is insoluble, infusible, and poor in processability, electrodes are formed via a sputtering process, or metallic electrodes are coated with polyamide acid being a precursor of polyimide, and then coated object is imidized to complete the process. Since complex facilities are needed to implement the above process, and yet, toxic gas is generated, specific facilities to purify toxic gas are needed, thus lacking in simplicity. And yet, since conventional polyimide film absorbs atmospheric water at a high rate, dielectric constant is variable, thus lowering reliability thereof.
In order to fully solve the above problems by way of presenting novel thermally-resistant thermoplastic resin and a variety of materials made from said novel resin being advantageous to effect presentation of tubular films, insulating materials, metallic-wire-covering materials, materials for electronic-circuit parts facilitating expansion of environment in which electronic apparatuses are usable, inventors have followed up overall studies and experiments and eventually found novel copolymer featuring distinct properties in terms of mechanical strength, resistivity against radioactive rays, chemical resistant property, stable characteristic under low temperature, thermal resistant property, processability, adhesive property, and stable thermal adhesive property simultaneously satisfying low hygroscopic characteristic and stable dielectric constant, and accordingly, concrete concept on film, powder particles, thermal fusible adhesive film for covering wires, laminated insulation material, electronic modules, and capacitors, has been consummated by way of applying novel thermoplastic polyimide resin mainly comprising said novel copolymer so that a variety of existing problems can fully be solved by effectively utilizing the above distinct physical properties.